Field of the Invention
This invention relates generally to electrolytes for lithium batteries, and, more specifically, to additives for solid block copolymer electrolytes.
Some block copolymer electrolytes include a “soft” block that provides ionic conductivity and a “hard” block that provides structural integrity. The resilience of polymer electrolytes based on such block copolymers is dependent on the thermomechanical properties of the so-called “hard” or “mechanical” block. Sufficient physical resistance to the intrusion of uneven or dendritic lithium growth when in contact with lithium metal electrodes is necessary to prevent penetration of lithium through the electrolyte and shorting of the cell. Some current block copolymer electrolytes use polyethylene oxide for the soft block and polystyrene for the hard block. Polystyrene is a well-characterized, inexpensive polymer with good properties, such as a high glass transition temperature (Tg, ˜100° C.), that allow it to be used at fairly high temperatures while maintaining high modulus values (>1 GPa), and good physical resiliency. Further improvements in either of these properties would be expected to increase battery performance by providing even greater physical resistance to lithium intrusion and by allowing operation at even higher temperatures, which would increase the ionic conductivity of the soft block.
There are a number of polymers that have better high-temperature properties than polystyrene and might be considered for use as the hard block in block copolymer electrolytes. But many of these are expensive engineering thermoplastics that may be difficult or impossible to use for forming block copolymers with poly(ethylene oxide) and/or to dissolve and process as a solution.
Another approach would be to crosslink the polymer used for the hard block. Cross-linking can improve physical properties, but requires great care in processing and perhaps additional processing steps for characterization and control. If a material crosslinks prematurely, it may not be possible for it to undergo further processing.